676 
fxus63 karx 071526 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
926 am CST Wednesday Jan 7 2009 


Update...this morning 


Northeast Iowa...southeast Minnesota...and western Wisconsin are 
currently in the left exit region of a jet moving through 
southern Minnesota. This has enhanced the vertical velocities 
and in turn a broad area of snow has developed across the 
aforementioned areas. This snow will move east across the region 
this morning. Snow amounts will be less than a half inch. 


&& 


Previous discussion... /issued 331 am CST Wednesday Jan 7 2009/ 


Short term...today through Saturday night 


Main forecast concern this package...small snow chances today into 
this evening...precipitation chances Thursday night through Friday night. 


Data analysis at 06z had a broad area of low pressure centered over 
the Ohio Valley with an inverted trough/front extending northwest 
into western WI/northeast Minnesota. Fog-product imagery and surface observation 
indicated clouds continuing to increase/thicken over much of 
Minnesota/IA/WI early this morning. More persistent snow was over southeast 
WI/northern Illinois under right jet entrance region. -Sn/flurries 
increasing over western Minnesota...east/ahead of shortwave energy 
dropping down the back side of the trough deepening over the region. 


No problems noted with 07.00z NAM/GFS initializations. Solutions 
quite similar through 48hrs...then GFS becomes faster than NAM with the 
trough moving east across the region in the 48-84hr time-frame. 
Non-NCEP models support NAM this time...with GFS appearing as the 
outlier in the 48-72hr time-frame. European model (ecmwf) trends toward compromise of 
nan/GFS by 84hrs. Dprog/dt of 500mb heights at 07.00z showed the 
NAM/GFS runs of 05.00z and 06.00z verified reasonably well across 
noam/ eastern Pacific with better run-to-run consistency to GFS. Through 
36hrs GFS shows the better run-to-run consistency as the trough 
deepens/moves across the region though trend is not as deep/closed 
with a low at 12-24hrs. For 36-60hrs between model and run-to-run 
consistency less than desirable for both NAM/GFS as the next 
trough/energy crosses the northern rockies. GFS strong/fast with the 
trough into the northern plains at 60hrs compared to NAM/non-NCEP 
models. GFS remains the faster solution with the trough passage at 
60-84hrs. European model (ecmwf) has trended faster in the 60-84hr period but not as 
fast as GFS. Check of observation vs model data at 06z showed NAM/GFS a bit 
weak with the inverted trough/low across WI. Per WV imagery...models 
appear to have a decent handle on the shortwave details in the 
central noam trough. No clear model favorite early on...with mixed 
signals for which one...if either...to favor based on trends/ 
consistency. European model (ecmwf) has shown some of the better consistency...as 
far as sensible weather late in the week. Thus favored a model 
blend/compromise weighted toward European model (ecmwf) this cycle. Given enough 
differences in the details and lesser run-to-run consistency among 
NAM/GFS...forecast confidence average this cycle. 


In the short term...deepening middle/upper level trough moves across 
the area today...but lift remains on the weak side. Model consensus 
has trended toward deeper moisture/saturation over the area today. 
This coupled with deep cyclonic flow and some convergence expected 
to result in -sn/flurries over the area...especially east of MS 
river...east of the middle/upper level trough axis. Probability of precipitation in the chance 
range appear reasonable...with the rather light precipitation event. Some 
deeper moisture/weak lift persists over the east end of the forecast 
area into the evening for flurry mention there. Thursday remains quiet 
with surface ridge axis and shortwave ridging aloft moving across the 
region. Models slowly converging on a common solution with the next 
shortwave trough impact the area Thursday night into Friday night. Strong 
850-700mb warm advection spreads across the area Thursday night. A good 
share of this GOES into warming with NAM/GFS indicating 850mb 
temperatures of 0c to +5c over the southwest half of the forecast area by 
12z Friday. NAM now the warmer of the models with this while European model (ecmwf)/ 
UKMET remain cooler...just sneaking 0c to +2c 850mb temperatures into the 
southwest corner of the forecast area at 12z Friday. Moisture... 
especially depth of saturation...remains an issue Thursday night/Fri. 
Models all indicating 3+ g/kg specific humidity available but deeper 
saturation through the dendritic growth zone is over roughly the 
northeast 2/3 of the forecast area later Thursday night/Friday in the colder 
airmass. Saturation looks problematic further west/south in the 
warmer airmass...with shallower saturation and top-down approach not 
producing ice crystals in the clouds. Moisture also appears too 
shallow for -fzra over the southwest end of the forecast area. 
Favoring a model compromise on 850mb warming by 12z Friday...carried 
mix of -fzdz/-pl/-sn in grids over the southwest third of the 
forecast area later Thursday night/Fri. As system GOES by Friday and 850mb 
layer cools...bulk of precipitation Friday afternoon looking to be snow but 
did not detail this for now. Did raise probability of precipitation some for later Thursday 
night/ Friday...mainly north of I-90 under the deeper saturation. Snow 
amounts will have to be watched for potential advisory amounts 
along/north of Highway 10. Favoring the slower model consensus Friday 
night...the middle/upper level trough axis is still west of the area at 
00z Sat. With deeper moisture and some lift remaining over the 
area...raised -sn chances Friday night. 


Numerical temperature guidance generally quite similar for today through Thursday 
night and looks reasonable. Still appears lows Thursday night to be 
during the evening with slowly rising temperatures overnight. Favored a 
blend of guidance highs for Friday. With more clouds expected Friday 
night...trended toward the warmer 07.00z GFS mex MOS lows. 


Long term...Saturday through Tuesday 


Model run-to-run consistency in the day 4-7 period remains good as 
far as the big picture of longwave ridging building into the Gulf of 
Alaska and and broad troughing over central noam. However much 
uncertainty remains in the details of shortwave passages in the 
increasing northwest flow aloft and cold/Arctic air intrusions 
behind them. Models continue to hint that a chunk of the polar 
vortex could slide across the area early to middle next week under the 
amplifying flow pattern and increasing northwest flow aloft. With 
some consensus for the first of the northwest flow shortwaves to 
drop across the area sun...added a small -sn chance. With plenty of 
uncertainty in the details beyond sun...made no changes to forecast 
grids for Mon/Tue. However...appears temperatures may need to be trended 
downward...especially by Tuesday. 


Aviation...today and tonight 


Weak cold front has move through both taf sites this morning. Two 
areas of MVFR ceilings around the area this morning. The first over 
much of Wisconsin shows some eastward progress and is expected to be 
out of the klse area by 07.12z. Second area is over western 
Minnesota also showing eastward progress. This area should spread 
over both taf sites between 12 and 18z. Once this shield of clouds 
moves in...there will be the potential for some light snow to fall 
but the probabilities of this producing an extended period of 
visibilities below 6 miles looks pretty low and do not plan to 
include any visibility restrictions. It appears that once the MVFR 
ceilings move in they should remain through 08.12z. 


&& 


Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 
&& 


$$ 


Short/long term...rrs 
aviation..........04 
update............boyne 

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